Look for a ferry ticket agent on Ios who has an agency for the ferry company you're using for Santorini-Rhodes - they can certainly sell you a ticket as it's all computerised. Blue Star Ferries who provide over half the service between Santorini and Rhodes will definitely have an agent on Ios as they have services to Ios as well.

It's normal in Greece for there to be a ticket office next to the ferry berth as well, for last-minute travellers.